Ultrasonication of toluene solutions of the heteropolynuclear cluster complex, Pt3Fe3(CO)15, in the presence of oleic acid and oleylamine affords surface-capped fcc FePt nanoparticles having an average diameter of ca. 2 nm. Self-assembled arrays of these nanoparticles on oxidized Si wafers undergo a fcc-to-fct phase transition at 775 degrees C to form ferromagnetic FePt nanocrystals ca. 5.8 nm in diameter well dispersed on the Si wafer surface. Room-temperature coercivity measurements of these annealed FePt nanoparticles confirm a high coercivity of ca. 22.3 kOe. Such high coercivity for fct FePt nanoparticles might result from use of a heterpolynuclear complex as a single-source precursor of Fe and Pt neutral atoms or from use of ultrasonication to form fcc FePt nanoparticles under conditions of exceptionally rapid heating. Experiments to determine the critical experimental conditions required to achieve such high room-temperature coercivities in ferromagnetic nanoparticles are underway.